Expert knowledge methods and systems for data analysis

ABSTRACT

A method for adjusting a data set defining a set of process runs, each process run having a set of data corresponding to a set of variables for a wafer processing operation is provided. A model derived from a data set is received. A new data set corresponding to one process run is received. The new data set is projected to the model. An outlier data point produced as a result of the projecting is identified. A variable corresponding to the one outlier data point is identified, the identified variable exhibiting a high contribution. A value for the variable from the new data set is identified. Whether the value for the variable is unimportant is determined. A normalized matrix of data is created, using random data and the variable that was determined to be unimportant from each of the new data set and the data set. The data set is updated with the normalized matrix of data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/698,400, filed on Jan. 25, 2007, which is a divisional of U.S.application Ser. No. 10/327,210, filed on Dec. 20, 2002, now U.S. Pat.No. 7,295,954 B2. The disclosures of these prior applications from whichpriority is claimed are incorporated herein by reference in theirentireties for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to methods for analyzing theperformance of wafer process operations run on wafer processingequipment, and more particularly, to methods for identifying variablesthat cause out-of-statistical-control signals and techniques forincorporating expert knowledge to ascertain the significance of suchsignals.

2. Description of the Related Art

In an attempt to quantify and study the affects of process conditionsduring wafer processing, process engineers are tasked with runningnumerous process runs, each with particularly set variables, and thencomprehensively studying results. The set variables, as is well know,are many. For instance, variables can include chamber pressure, chambertemperature, the delivered power to one or both electrodes,electrostatic chuck clamping voltage, types of gases and flow rates,etc. In practice, therefore, data for such variables is measured andrecorded as wafers are put through process runs. Databases are createdto organize the data for such variables. In the analysis of such data,specific attention is paid to ascertain whether the behavior ofparticular variables is within an acceptable range.

Multivariate statistical process control tools are available for themonitoring of deviations between historical process runs and new processruns. These tools can statistically define normal operating behavior ina process based on historical data. Statistical projection-basedtechniques, such as principal component analysis (PCA), are used toproduce out-of-statistical-control signals when a variable is identifiedas deviating out of the bounds of normal operating behavior.

As multivariate statistical process control tools accommodate analysisacross a large number of variables the resulting models are verysensitive, too sensitive with respect to some variables.

Another challenge associated with using these techniques is to determineif an out of bounds signal is considered meaningful based on expertknowledge. Some variables or ranges of variable values are more criticalthan others. For example, once a wafer is clamped into position theclamp voltage could vary, yet still not be considered a fault, or errorin the system. Generally, faults are generated when a value for avariable changes so much that it falls out-of-statistical control. So,if a value for clamp voltage is recorded as being out of an acceptablestatistical bounds relative to other variables in the system, it may beflagged as a problem and an automatic fault code would be sent outhalting the wafer processing.

However an expert observing the same value for clamp voltage might notbe concerned with the variable deviation. For example, though the valuefor clamp voltage is out of the acceptable statistical bounds it couldstill fall in an operating range where the clamp properly holds thewafer. Unfortunately, a fault would still be registered, even thoughexpert knowledge would deem the out of bounds signal as not warranting afault. The end result is that reliance on pure mathematical statisticalanalysis will lead to false fault alarms. Nevertheless, duringprocessing, every fault will generally lead to stoppage of waferprocessing operations, thus resulting in wasted time and money.

The models generated in statistical projection-based techniques can bemade more robust by incorporating large amounts of data for a particularprocess and by incorporating detailed information for each variablebeing recorded. The limitation with this approach is that during thephase when models are being built large amounts of data are not alwaysavailable for the variables and the cost of experimental operation canbe very impractical.

In view of the foregoing, what is needed is a method and system forincorporating expert knowledge in the identification and reduction offalse fault alarms in wafer processing systems.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills this need by providing amethod and system for incorporating expert knowledge for theidentification of unimportant out-of-statistical-control signals inwafer processing systems. Several embodiments of the invention aredescribed below.

In one embodiment, a method for adjusting a data set defining a set ofprocess runs, each process run having a set of data corresponding to aset of variables for a wafer processing operation is provided. A modelderived from a data set is received. A new data set corresponding to oneprocess run is received. The new data set is projected to the model. Anoutlier data point produced as a result of the projecting is identified.A variable corresponding to the one outlier data point is identified,the identified variable exhibiting a high contribution. A value for thevariable from the new data set is identified. Whether the value for thevariable is unimportant is determined. A normalized matrix of data iscreated, using random data and the variable that was determined to beunimportant from each of the new data set and the data set. The data setis updated with the normalized matrix of data.

In another embodiment, a method for adjusting a data set defining a setof process runs, each process run having a set of data corresponding toa set of variables for a wafer processing operation is provided. A modelderived from a data set is received. A new data set is received. The newdata set is projected to the model. Outlier data points produced as aresult of the projecting are identified. One of the outlier data pointsfrom the outlier data points is identified. A variable corresponding tothe one outlier data point is identified, the identified variableexhibiting a high contribution. Whether the variable is unimportant isdetermined. A normalized matrix of data is created, using data from thenew data and from the data set, the normalized matrix of data createdusing the variable that was determined to be unimportant from each ofthe new data and the data set. The data set is updated with thenormalized matrix of data.

In accordance with another aspect of the present invention, a method forupdating a data set defining a set of process runs, each process runhaving a set of data corresponding to a set of variables for a waferprocessing operation is provided. A data set is received. Scaling to thedata set is performed. Principal component analysis is performed to thescaled data set to generate a model. New data is received. The new datais projected to the model. Outlier data points based on the projectingare identified. A contribution plot corresponding to one of the outlierdata points is examined. A variable that corresponds to the one outlierdata point which provides a high contribution in the contribution plotis identified. That the variable is unimportant is determined. Adesensitizing set of data for the variable is created based on astandard deviation of the data set and a randomization of the new data.The data set is augmented with the desensitizing set of data.

In one embodiment, a method for adjusting a data matrix defining a setof process runs each process run having a set of data corresponding to aset of variables for a wafer processing operation is provided. A datamatrix of N rows and M columns where N equals a number of process runsand M equals a number of variables in the data matrix is received. A newset of data with M variables wherein at least one variable correspondsto an outlier and is unimportant based on expert input is received. Anormally distributed random vector containing N−1 rows is generated. Aone vector containing N−1 rows of ones is generated. A standarddeviation of data corresponding to the variable in the data matrix isdetermined. The standard deviation is multiplied by the normallydistributed random vector producing a first vector. The datacorresponding to the variable from the new data is multiplied by the onevector producing a second vector. The first vector is added to thesecond vector producing a third vector. An expert desensitizing matrixis created where the Mth column contains the third vector and theremaining columns are made up of data corresponding to the remainingvariables. A new data matrix is created where the data matrix isaugmented by the expert desensitizing matrix.

The advantages of the present invention are numerous. One notablebenefit and advantage of the invention is that data sets of process runsin wafer process systems can be desensitized incorporating expertknowledge to unimportant variable data by incorporating smaller amountsof data.

Other advantages of the invention will become apparent from thefollowing detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

It is to be understood that the foregoing general description and thefollowing detailed description are exemplary and explanatory only, andare not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute partof this specification, illustrate exemplary embodiments of the inventionand together with the description serve to explain the principles of theinvention.

FIG. 1 is a flow chart diagram 100 illustrating the method of operationsperformed to adjust a data set of process runs for a wafer processingsystem, in accordance with one embodiment of the invention.

FIG. 2 provides a residual plot and one accompanying variablecontribution plot, in accordance with one embodiment of the invention.

FIG. 3 provides two residual plots, in accordance with one embodiment ofthe invention.

FIG. 4 provides a variable contribution plot, in accordance with oneembodiment of the invention.

FIG. 5 provides a correlation coefficient chart, in accordance with oneembodiment of the invention.

FIG. 6 provides a comparison of the structure of the original model andthe model desensitized in accordance with one embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Several exemplary embodiments of the invention will now be described indetail with reference to the accompanying drawings.

FIG. 1 is a flow chart diagram 100 illustrating the method of operationsperformed to adjust a data set of process runs for a wafer processingsystem so as to desensitize the data set to outliers, which areidentified as unimportant variable data, in accordance with oneembodiment of the invention. The method begins with operation 110, inwhich a data set is received. For example, in a wafer processing systemmany parameters or variables are tracked, such as, for example, chamberpressure, chamber temperature, the delivered power to one or bothelectrodes, electrostatic chuck clamping voltage, types of gases andflow rates, etc. In fact, the variables can include any variable thatcan be recorded, or gets changed due to an impact of a hardware changeor software change. Thus, the variables can represent a range ofvariables defining changes in a design of equipment used to performprocess runs. And, as each of a series of wafers is put through aprocess run the data for each variable in the wafer processing system ismeasured and recorded. The data set, in this embodiment, will refer todata corresponding to a set of variables for a series of process runs ina wafer processing system.

Once the data set is received, the method advances to operation 112, inwhich scaling is performed on the data set. In one embodiment, the dataset can be autoscaled such that each variable is in standard units(i.e., has a zero mean and a unit standard deviation) to ensure that thedata can be compared across variables with different units. Assuming thedata set has been scaled, the method advances to operation 114.

In operation 114, principal component analysis (PCA) is performed on thescaled data set to produce a PCA model and multivariate statisticalnumbers (such as, for example, Q-residual, T² residual) of the data set.Multivariate statistical process control is used to define normaloperating behavior in a process by statistically analyzing the data fromthe process. Principal component analysis (PCA) is a statisticalprojection-based technique, which enables the detection ofout-of-statistical-control signals in a process. PCA provides astatistical evaluation of the data set. Modeling methods used formultivariate statistical process control, other than PCA, could be usedin this step as well. The PCA performed on the scaled data set willprovide a statistical evaluation of the data set that includes a PCAmodel.

After the principal component analysis has been performed on the scaleddata set, the method advances to operation 116. In operation 116 a modelis received. The model received is the PCA model which is a result ofthe PCA operation performed on the data set in operation block 114. Oncethe model is received, the method advances to operation 118.

In operation 118, a new data set is received. The new data set receivedin operation 118 is new data corresponding to one process run in a waferprocessing system having the same set of variables as the original dataset received in operation 110. This method could also be used for a newdata set composed of more than one process run. Once the new data set isreceived, the method advances to operation 120.

In operation 120, the new data set is scaled and projected to the model.In the embodiment shown in FIG. 1, the new data set is scaled using thesame mean and standard deviation of the variables of data set before thenew data is projected onto the model.

The projection of the new data set to the model will provide a residualplot with corresponding variable contribution plots. These plots willprovide statistical information regarding the relationship between thenew data set from operation 118 and the data set from operation 110. Theresidual plot will show if the new data set for the new process run fromoperation block 118 corresponds to a statistical outlier relative to themultivariate mean, multivariate variation, and a chosen confidencebounds for the data set from operation 110. In this embodiment, theresidual plot used is the Q-residual plot. In another embodiment, theresidual plot used is the T² residual plot.

Assuming the new data set has been projected to the model, the methodadvances to operation 122. In operation 122, the residual plot producedwhen the new data set was projected to the model is examined todetermine if an outlier data point or points exist. For a new data setcontaining more than one process run, each process run will correspondto a single data point on the residual plot. If the new data set pointfalls out of an acceptable range of the Q-residual of the data set fromoperation 110 (i.e., is out of bounds with respect to the modelconfidence limit), then an outlier exists. Once the determinationwhether an outlier exists has been made, the operation will proceed toeither operation block 124 if an outlier does not exist or to operationblock 126 if an outlier does exist.

If an outlier does not exist in the residual plot examined in operationblock 122, the method advances to operation 124. In operation 124, thenew data set is incorporated with the data set received in operation 110to create an expanded data set. This expanded data set is then sent tooperation block 110 and the method begins again from operation block110.

If an outlier does exist in the residual plot examined in operationblock 122, the method advances to operation block 126. In operation 126,the contribution plot that corresponds to the outlier data pointidentified in operation 122 is examined for a variable contributor witha high contribution. The residual plot produced after the new data setis projected onto the model in operation 120 will contain one data pointfor each process run of the new data set. Each data point on theresidual plot will then in turn have an associated contribution plot.Each contribution plot will show the relative contribution for eachvariable's contribution to the Q-Residual. If a variable has a highrelative contribution then it is identified as a variable contributorwith a high contribution. Whether a contribution is high is a subjectivedetermination. For this example we will assume that there is only onevariable contributor with a high contribution. However the contributionplot can have more than one variable contributor and can be accommodatedby the method as will be discussed below. Once the contribution plot isexamined and a variable is identified as providing a high contributionthe method advances to operation block 128.

In operation 128, expert knowledge is used to determine if the variableor the value of the variable identified as providing a high contributionis considered unimportant. Here expert knowledge is informationregarding the variables or the value of the variables. An expert is, inthis example, an engineer that has extensive knowledge and experienceregarding the significance of each variable. Thus, if the variablecontributor is determined to be, for example, the wafer clamp voltagevariable, the expert might consider this variable to be an unimportantvariable.

Furthermore, the expert knowledge might deem a certain range of valuesfor wafer clamp voltage to be acceptable, even if the value is out ofacceptable statistical bounds. For example, the wafer clamp voltagecould be within engineering specifications. Therefore a fault should notbe called in the system based on the given value for the wafer clampvoltage. In this case, the expert knowledge would label a particularrange of values for the variable as unimportant. If the value of thevariable is considered by the expert knowledge to actually necessitatecausing a fault, then after operation block 128 the method will advanceto operation block 130. In operation block 130, the detected out ofstatistical bounds variable will trigger a fault. Alternatively, if theexpert knowledge labels the variable or the value of the variable asunimportant, then the method will proceed to operation block 132.

In operation 132, an expert randomizer (ER) will be implemented tocreate a desensitizing matrix, and then the desensitizing matrix will beaugmented to the data set originally received in operation block 110.The expert randomizer implements a method that normalizes the new datacorresponding to the variable by using normally distributed random dataand the variable data from the data set along with the variable datafrom the new data. In the embodiment shown in FIG. 1, the data set andthe new data set represent the data as received in operation blocks 110and 118, respectively (i.e., before scaling). The expert randomizer isconfigured to generate a normally distributed desensitizing matrix usingrandom data and the variable data (i.e., values for the variable beingdesensitized). The desensitizing matrix is then augmented to the dataset received in operation block 110.

The desensitizing matrix will desensitize the data set with respect tothe value of the variable in the new data, such that if the samevariable data is seen in a subsequent set of new data the same variabledata will not cause an outlier in a residual plot when the new data isprojected onto the PCA model. Utilizing the teachings of this inventionis significantly improved over prior art embodiments that performtrivial desensitization by removing a variable or variables that causethe data to be an outlier from the analysis entirely. This is animportant distinction, as the removal of variables eliminates theability to continue detecting outliers based on those particularvariables. According to the teachings of this invention, such outlierscan still be detected at levels of deviation above the desensitizationlevel, which is still very valuable.

One of the strengths of multivariate statistical models for faultdetection is the quantification of fault behaviors involving correlatedchanges amongst the variables. These correlations and their relativesensitivities to variable deviations are referred to as the “structure”of the model which can be visually assessed by comparing the loads ofthe variables in the model. Another advantage inherent in the methods ofthis invention is that the model is desensitized to deviations in avariable or variables without destroying the structure of the model,which can be seen in FIG. 6. As a result, the benefits of the originalmultivariate model are maintained to detect the same types of correlatedchanges amongst the variables. The expert randomizer is one embodimentof a method to normalize the variable data. More details for the expertrandomizer will be provided below.

After the data set originally received in operation block 110 isaugmented by the desensitizing matrix, the resulting matrix of data willbe sent to operation block 110 to begin the method again if necessary.

An example illustrating the methodology described above as applied to anew data set having more than one process run will be discussed next,with reference to FIG. 2. In this example, the original data, i.e., thedata set received in operation block 110, contains 45 samples (i.e.,process runs) with 29 variables each. The new data, i.e., the new dataset received in operation block 118 contains 44 samples with 29variables each. Assuming the data set is properly scaled, principalcomponent analysis is performed to create a model. Next, the new dataset is projected to the model. After the new data set is projected tothe model a residual plot is produced, and from the residual plot it canbe determined if one of the 44 process runs represented in the new datahas caused an outlier.

As shown in FIG. 2, a residual plot and one accompanying variablecontribution plot is provided, in accordance with one embodiment of theinvention. As described above, FIG. 2 provides a residual plot 140associated with projecting a new data set of 44 process runs (with 29variables each) to a PCA model and a variable contribution plot 170,corresponding to the exemplary 44^(th) process run. The horizontaldashed line 150 intersecting the origin represents the model confidencelimit determined from the original data set. The line 155 representedwith a line intersecting the circles shown represents the Q residualvalues for the 44 process runs of the new data. As shown in the residualplot 140, all of the 44 process runs of the new data set are outlierswith respect to the data set.

The variable contribution of the exemplary 44^(th) process run is shownin variable contribution plot 170. The relative contribution of variablefour (4) 175 is high. For this example there is only one variable with ahigh contribution, but in other embodiments there could be more than onevariable being a high contributor. After variable four has beenidentified in contribution plot 170 as having a high contribution,expert input will be sought to determine if variable four is unimportant(i.e., if the value for variable four, though causing an outlier, isstill acceptable and should not be labeled as a fault). Assuming thatvariable four is deemed as unimportant based on expert knowledge, thenew data set will be normalized. Here, an expert randomizer is used toadjust the new data set. One embodiment of an expert randomizer will beprovided in more detail below.

Once the expert randomizer has been applied to create a desensitizingmatrix and the data set is augmented with this desensitizing matrix themethod can be run again with the augmented data set. If the method isrun again with a new data set having the same values as the first newdata discussed above, the variable four data will not cause an outlier.The outlier behavior of the variable four data will be desensitized outof the data set. The result of running the method again using the dataset augmented with the desensitizing matrix, and introducing a new dataset identical to the 44 process run data set used above, will be shownin a side-by-side residual plot.

FIG. 3 provides two residual plots, in accordance with one embodiment ofthe invention. FIG. 3 provides the residual plot from FIG. 2 at the 0 to44 sample number side of the graph, where a sample number indicates theprocess run. The remaining section of the graph provides a residual plotresulting from executing the method a second time. In the secondexecution of the method the data set has been augmented with thedesensitizing matrix and the new data set used is identical to the 44process run data set used above. In comparing the left side of thegraph, showing the residual plot representing the new data without theexpert knowledge, to the right side of the graph showing the residualplot representing the new data after the expert knowledge has beenimplemented by way of the desensitizing matrix, it is shown that theresidual of the 44 sample set of data has been significantly reduced andthe analysis significantly desensitized. The second data run is nowwithin the model confidence limit. The model confidence limit is thesame as horizontal dashed line 150 in FIG. 2, representing the data seton the PCA model projection.

FIG. 4 provides a variable contribution plot, in accordance with oneembodiment of the invention. The variable contribution plot in FIG. 4represents the 44^(th) sample accompanying the residual plot thatresults from running the method using the data set augmented with thedesensitizing matrix, and then introducing a new data set identical tothe 44 process run data set used initially. As shown in FIG. 4, variablefour is no longer a high contributor. So the expert randomizer hasconditioned the data set such that the variable four data that initiallycaused an outlier is now considered statistically normal.

The embodiment of the invention works more efficiently if the variableat issue, such as variable four above, does not have a strongcorrelation with the other variables in the process run. The correlationacross the variables can be checked with the use of a correlationcoefficient chart. A correlation coefficient chart is provided for theexample discussed in FIGS. 2 and 3. Correlation coefficient charts canbe generated with the application of PCA.

FIG. 5 provides a correlation coefficient chart, in accordance with oneembodiment of the invention. As shown in FIG. 5, variable four is notstrongly correlated with the other 28 variables in the process run forsample 44. If the correlation between variable four and other variableswere strong, then the method might be more difficult to apply. Whenapplying the expert randomizer to a variable of interest, if othervariables are strongly correlated to the variable of interest they willalso be desensitized, to some extent. So, if there are variables thatare strongly correlated to a variable of interest and these variablesshould not be desensitized, the expert randomizer will be lesseffective. Therefore before the expert randomizer is applied to a newdata set, it is valuable to review the correlation charts for how thevariable of interest correlates with the remaining variables in a dataset.

Exemplary code for an expert randomizer is provided in the followingtables. In Table 1 below, the code provides for the case when the newdata set is composed of one sample, i.e., one process run. Table 2 belowprovides for the case when the new data set is composed of multiplesamples, i.e., more than one process run. Both Table 1 and Table 2provide for the case when one or more variables is identified to bedesensitized within a single process run of new data. The matlabvariable definitions are provided in the Notations section. Note thatalthough matlab is used, any appropriate software language to carry outthe functionality could be used to create an expert randomizer.

TABLE 1 Notations: Data: matrix containing original data which isimported m: number of samples in the original data n: number ofvariables in the original data x: matrix containing new data, which isimported mm: number of samples in the new data having same number ofvariables (n) p: index of variables that are determined as unimportantand have high contributions kn: number of variables to discard that arenot correlated strongly to other variables km=1, number of rows fordiscarded vector s: standard deviation of the variables in the originaldata, which is imported NDR: normally distributed random vector of sizem-by-1 O: vector of ones of size m-by-1 r: column vector that is beingcalculated based on expert knowledge ED: Expert Data matrix EXPERTRANDOMIZER MATLAB ROUTINE Case1: Exemplary code for one sample of newdata ED=zeros(m,n); % assigning/initializing ED to be a matrix of zerosof size m-by-n ED(1,:)=x; % assigning 1^(st) row of ED as the new samplefor j=1:kn % starts the loop for generating an expert data matrix foreach variable to be desensitized NDR=randn(m−1,1); % generating anormally distributed random column-vector O=ones(m−1,1); % generating acolumn-vector of ones for computation r(:,j)=s(1,p(j))*NDR +x(mm,p(j))*O; % calculating the new value of the variable to bedesensitized based on expert knowledge if p(km,j)>1 % checking if thevariable to be discarded is not the first variable in the matrix fori=2:m % loop for replicating the samples ED(i,1:p(j)−1)=x(mm,1:p(j)−1);ED(i,p(j)+1:end)=x(mm,p(j)+1:end); end elseif p(km,j)==1 % checking ifthe variable to be discarded is the first variable in the matrix fori=2:m ED(i,p(j)+1:end)=x(mm,p(j)+1:end); % assigning the values ofvariables not being desensitized end end end for pp=1:knED(2:m,p(pp))=r(:,pp); % assigning the values of variables beingdesensitized end Data=[Data;ED]; % augmenting the original data matrixto include the new matrix

TABLE 2 Notations: Data: matrix containing original data, which isimported m: number of samples in the original data n: number ofvariables in the original data x: matrix containing new data mm: numberof samples in the new data having same number of variables (n) p: indexof variables that are determined as unimportant and have highcontributions kn: number of variables to discard that are not correlatedstrongly to other variables km=1, number of rows for discarded vector s:standard deviation of the variables in the original data, which isimported NDR: normally distributed random vector of size m-by-1 O:vector of ones of size m-by-1 r: column vector that is being calculatedbased on expert engineering knowledge ED: Expert Data matrix EXPERTRANDOMIZER MATLAB ROUTINE Case2: Exemplary code for any number ofsamples of new data. ED=zeros(2*mm,nn); % assigning/initializing ED tobe a matrix of zeros of size m-by-n ED(1:mm,1:nn)=x; % assigning firstrow of ED as the new sample for j=1:kn % starts the loop for generatingan expert data matrix for each variable to be desensitizedNDR=randn(mm,1); % generating a normally distributed randomcolumn-vector O=ones(mm,1); % generating a column-vector of ones forcomputation r(:,j)=s(1,p(j))*NDR + x(mm,p(j))*O; % calculating the newvalue of the variable to be desensitized based on expert knowledge ifp(km,j)>1 % checking if the variable to be discarded is not the firstvariable in the matrix for i=mm+1:2*mm % loop for replicating thesamples ED(i,1:p(j)−1)=x(i-mm,1:p(j)−1);ED(i,p(j)+1:end)=x(i-mm,p(j)+1:end); end elseif p(km,j)==1 % checking ifthe variable to be discarded is the first variable in the matrix fori=mm+1:2*mm ED(i,p(j)+1:end)=x(i−mm,p(j)+1:end); % assigning the valuesof variables not being desensitized end end end for pp=1:knED(mm+1:2*mm,p(pp))=r(:,pp); % assigning the values of variables beingdesensitized end Data=[Data;ED]; % augmenting the original data matrixto include the new matrix

Provided below is an example describing the execution of the ExpertRandomizer Matlab Routine provided in Table 1. In this example the newdata set includes one sample (i.e., process run). Only one variable willbe desensitized in this example. The example is generalized to provideand overall understanding of operation steps performed in the ExpertRandomizer Matlab Routine provided in Table 1. The example may notinclude a description of every operation of the code.

Step 1

Certain matlab variables are assigned values by importing data(initializations not all shown in code). Data is the matlab variablecontaining the original data for the wafer process system. The values ofthe original data are already known based on previous runs for the sameprocess on the same equipment. The original data is imported into matlabfor use in the calculations. In this example the value for Data follows:

Data = Column 1 2 3 Row 1 10.0 200.0 3.2 Row 2 10.1 200.1 3.2 Row 3 9.8199.8 3.1 Row 4 10.0 200.2 3.1

Data is represented by a matrix of m (=4) rows and n (=3) columns. Eachcolumn represents a variable in the wafer processing system (i.e., thereare 3 variables in this example). Each row represents a sample (i.e.,process run).

Step 2

The new data is represented by the matlab variable x. The new data mustcontain the same number of variables (n=3) as the original data. The newdata is imported into matlab and initialized. For this example the valuefor each variable/column for the new data is:

x=10 200 3

Before the Expert Randomizer is initiated it is assumed that x has beendetermined to be an outlier (when compared to the original data,‘Data’). Also, for this example the third variable has been assumed tobe a high contributor and has been labeled as unimportant based onengineering knowledge.

Step 3

ED is initialized having the same size as the Data matrix (step 1),where each element is assigned a value of 0.

ED=zeros(m,n)

For this step the value for ED is:

ED = 0 0 0 0 0 0 0 0 0 0 0 0

Step 4

The first row of ED is assigned the values from the new sample, x Forthis step the value for ED is:

ED = 10 200 3 0 0 0 0 0 0 0 0 0

Step 5

The standard deviation for Data is calculated. This standard deviationwill be used to generate the expert randomizer.

s=std(Data)

std is a matlab command that calculates the standard deviation of amatrix.So, the value for s is shown below:

s=0.13 0.17 0.06

Each number in the s vector denotes the standard deviation for avariable in the Data matrix.

Step 6

Next, a normally distributed random vector with 3 rows is generated.

NDR=randn(3,1), where randn is a standard matlab command that generatesrandom data.

The value for NDR is shown below:

NDR=

0.7119

1.2902

0.6686

Step 7

A vector labeled O is generated. The O vector contains ones and has thesame number of rows as NDR. This vector is used to facilitate with thematrix computations.

O=ones(3,1), where ‘ones’ is a standard matlab command

The value for O is shown below:

O=

1

1

1

Step 8

In this step the standard deviation of the third variable from Data(i.e., s(1,3)) is multiplied by NDR. The resulting value is added to thevalue for the third variable from the new data (x(1,3)).

r=s(1,3)*NDR+x(1,3)*O

This provides:r=

3.0411

3.0745

3.0386

Step 9

Multiple copies of the new data, which is not being desensitized, aremade and assigned to the second, third and fourth rows of the ED matrix

ED = 10 200 3 10 200 0 10 200 0 10 200 0

Step 10

The value for r is assigned to rows 2 through 4 of the third column ofthe ED matrix.

ED (2:4,3)=r

Therefore,

ED = 10 200 3.0000 10 200 3.0411 10 200 3.0745 10 200 3.0386

Step 11

Finally, ED is augmented to the Data matrix. This new matrix replacesData. Data=[Data;ED]

Therefore,

Data = 10.0 200.0 3.2000 10.1 200.1 3.2000 9.8 199.8 3.1000 10.0 200.23.1000 10.0 200.0 3.0000 10.0 200.0 3.0411 10.0 200.0 3.0745 10.0 200.03.0386

As described, the Expert Randomizer provides a method for adjusting anew data set, which has met the outlier, high contribution, and expertknowledge criteria discussed above to then normalize the new data withrespect to the original data set. The normalized data is then augmentedto the original data set. So, the augmented data will have beendesensitized to the variable data which initially caused an outlier.

Any of the operations described herein that form part of the inventionare useful machine operations. The invention also relates to a device oran apparatus for performing these operations. The apparatus may bespecially constructed for the required purposes, or it may be ageneral-purpose computer selectively activated or configured by acomputer program stored in the computer. In particular, variousgeneral-purpose machines may be used with computer programs written inaccordance with the teachings herein, or it may be more convenient toconstruct a more specialized apparatus to perform the requiredoperations.

The invention can also be embodied as computer readable code on acomputer readable medium. The computer readable medium is any datastorage device that can store data which can thereafter be read by acomputer system. Examples of the computer readable medium include harddrives, network attached storage (NAS), read-only memory, random-accessmemory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and other optical andnon-optical data storage devices. The computer readable medium can alsobe distributed over a network coupled computer systems so that thecomputer readable code is stored and executed in a distributed fashion.

It will be further appreciated that the instructions represented by theoperations in FIG. 1 are not required to be performed in the orderillustrated, and that all the processing represented by the operationsmay not be necessary to practice the invention. Further, the processesdescribed in FIG. 1 can also be implemented in software stored in anyone of or combinations of the RAM, the ROM, or the hard disk drive.

In summary, the embodiments of the present invention provide a methodfor incorporating expert knowledge for the identification of unimportantout-of-statistical-control signals in wafer processing systems. Theinvention has been described herein in terms of several exemplaryembodiments. Other embodiments of the invention will be apparent tothose skilled in the art from consideration of the specification andpractice of the invention. The embodiments and preferred featuresdescribed above should be considered exemplary, with the invention beingdefined by the appended claims and equivalents thereof.

1. A method for updating a data set defining a set of process runs, eachprocess run having a set of data corresponding to a set of variables fora wafer processing operation, comprising: receiving a data set;performing scaling to the data set; performing principal componentanalysis to the scaled data set to generate a model; receiving new data;projecting the new data to the model; identifying outlier data pointsbased on the projecting; examining a contribution plot corresponding toone of the outlier data points; identifying a variable that correspondsto the one outlier data point which provides a high contribution in thecontribution plot; determining that the variable is unimportant;creating a desensitizing set of data for the variable based on astandard deviation of the data set and a randomization of the new data;and augmenting the data set with the desensitizing set of data.
 2. Themethod of claim 1, wherein determining that the variable is unimportantis performed with expert knowledge.
 3. The method of claim 2, whereinexpert knowledge is knowledge of behavior of the variable.
 4. A methodfor adjusting a data matrix defining a set of process runs each processrun having a set of data corresponding to a set of variables for a waferprocessing operation, comprising: receiving a data matrix of N rows andM columns where N equals a number of process runs and M equals a numberof variables in the data matrix; receiving a new set of data with Mvariables wherein at least one variable corresponds to an outlier and isunimportant based on expert input; generating a normally distributedrandom vector containing N−1 rows; generating a one vector containingN−1 rows of ones; determining a standard deviation of data correspondingto the variable in the data matrix; multiplying the standard deviationby the normally distributed random vector producing a first vector;multiplying the data corresponding to the variable from the new data bythe one vector producing a second vector; adding the first vector to thesecond vector producing a third vector; creating an expert desensitizingmatrix where the Mth column contains the third vector and the remainingcolumns are made up of data corresponding to the remaining variables;and creating a new data matrix where the data matrix is augmented by theexpert desensitizing matrix.
 5. An expert system for desensitizingvariables based on engineering knowledge, comprising: a first databasethat includes data for process runs; a second database that includescorresponding models of the data; a processor coupled to the first andsecond databases; and logic that identifies outliers and variablecontributions that caused the outliers, the logic being furtherconfigured to incorporate expert engineering knowledge in an examinationof the variable contributions, and the logic adjusts the data in orderthat future process runs properly identify resulting outliers as faults.6. An expert system for desensitizing variables based on engineeringknowledge as recited in claim 5, wherein the expert system enablesproper fault detection due to a desensitizing of variable contributionsthat can cause false positive faults.
 7. An expert system fordesensitizing variables based on engineering knowledge as recited inclaim 5, wherein the variables represent a range of variables definingchanges in a design of equipment used to perform the process runs.